package leetcode;

/**
 * http://leetcode.com/onlinejudge
 * 
 * Climbing Stairs
 * 
 * You are climbing a stair case. It takes n steps to reach to the top.
 * 
 * Each time you can either climb 1 or 2 steps. In how many distinct ways can
 * you climb to the top?
 * 
 * @author yunzhang
 * 
 */
public class ClimbingStairs {
	/**
	 * Solves the problem with DP.
	 * 
	 * @param n
	 * @return
	 */
	public int climbStairs(int n) {
		if (n < 1)
			return 0;
		if (n == 1)
			return 1;

		int[] ways = new int[3];

		ways[0] = 1;
		ways[1] = 1;

		for (int i = 2; i <= n; ++i)
			ways[i % 3] = ways[(i - 1) % 3] + ways[(i - 2) % 3];

		return ways[n % 3];
	}
}
